The present invention relates to a hybrid (hereafter abbreviated as HB) permanent magnet type electric rotating machine used for OA equipment treating images, such as a facsimile, an ink-jet printer, a laser beam printer, or a copy machine. The present invention also relates to a manufacturing method of such an electric rotating machine.
FIG. 14 and FIG. 15 show a conventional two-phase HB type stepping motor. FIG. 14 is a longitudinal sectional view and FIG. 15 is a front view showing a relation between a rotor core and a stator core. This stepping motor 100 is provided with a stator 110 having a stator core 111 of two-phase and eight-pole, and a rotor 130 fixed to a rotation shaft 120 that is rotatably supported inside the stator 110. The stator 110 is constructed by sandwiching a stator core 111, which is formed by stacking silicon steel plates, by a pair of brackets 112 and 113 formed from nonmagnetic material from the both sides in the axial direction. An excitation winding 114 wound around a bobbin 115 is attached to each main pole 111a formed on the stator core 111. As shown in FIG. 15, the stator core 111 includes the eight main poles that are radially formed to be directed to the center of the stator core 111. A plurality of inductors 111b are formed on the tip portion of each main pole 111a. 
The rotor 130 is constructed by sandwiching a disc-like permanent magnet 133, which is magnetized in the axial direction, by a pair of rotor cores 131 and 132. As shown in FIG. 15, many small teeth 130a are formed around one rotor core 131. The same number of small teeth are formed around the other rotor core 132. The rotor cores 131 and 132 are fixed to the rotation shaft 120 so that they are deviated by ½ pitch of the small teeth. References 121 and 122 denote bearings.
As shown by a phase A, a phase B, a phase A′, and a phase B′ in FIG. 15, the excitation coils of one phase are wound around every other four main poles of the eight main poles 111a of the stator core 111. In this case, since the main poles located at the opposite positions at 180 degrees are excited in the same polarity when the excitation current is applied, the attraction forces in the radial direction are always canceled, the torque component in the tangential direction of the outer circumference of the rotor appears.
However, in the above-mentioned conventional structure (a full-main-pole structure), the stator has many main poles, and the manufacturing cost becomes high. In a reduced-main-pole structure (a half-main-pole structure) in which a stator has four main poles, the rotor core 132 is pulled in the lower direction as S polarity, when the rotor core 131 is pulled in the upper direction as N polarity, for example. This generates an unbalanced electromagnetic force due to the attraction forces in the radial direction (couple of forces due to so-called side pull), which generates vibration and noise, and deteriorates positioning accuracy.
On the other hand, U.S. Pat. No. 6,781,260 discloses a stepping motor of the reduced-main-pole structure (the half-main-pole structure) that has four main poles with large torque and low vibration. The stepping motor disclosed in the publication is constructed by arranging a rotor having two sets of rotor units inside a stator on which coils are wound. Each of the rotor units consists of a ring-shaped unipolar permanent magnet whose flat surfaces are magnetized and a pair of rotor cores that sandwich the permanent magnet. Many small teeth are formed around each of the rotor cores. The two rotor units are attached to a motor shaft so that the magnets have opposite polarities to make the magnetic polarities of the rotor teeth of the adjacent two rotor cores identical. Since the stepping motor of the publication is provided with four rotor cores, the radial attraction forces are distributed and balanced as compared with a conventional motor with two rotor cores. Therefore, an unbalance moment does not occur, which reduces vibration and noise owing to clearances of bearings or the like. That is, vibration and noise are lower than the conventional motor. This stepping motor theoretically generates double the torque of the same-size conventional motor of the full-main-pole structure shown in FIG. 14 and FIG. 15. Alternatively, when this stepping motor is designed so as to obtain the same torque as the motor of the full-main-pole structure, an air gap between the stator and the rotor can be larger, which reduces fraction defective and improves reliability. Since this stepping motor can use a cheap magnet such as a ferrite magnet, the manufacturing cost becomes lower than the conventional full-main-pole motor that uses a rare earth permanent magnet with high energy.
Although the motor structure disclosed in U.S. Pat. No. 6,781,260 requires that two permanent magnets are magnetized in the opposite polarities, an appropriate magnetization method has not been established. A permanent magnet is magnetized by a magnetic flux whose density reaches its saturation flux density. However, when a conventional magnetizing device with an air-core coil tries to magnetize two permanent magnets at the same time, the permanent magnets are insufficiently magnetized because the magnetic fluxes for magnetization repel to each other. On the other hand, when the conventional magnetizing device tries to magnetize two permanent magnets one by one with time difference, the magnetic flux for magnetizing the permanent magnet of one rotor unit leaks to the other rotor unit. The leakage magnetic flux may magnetize the permanent magnet of the other rotor unit in the polarity opposite to the desired polarity, or may demagnetize the permanent magnet of the other rotor unit that has been already magnetized. Conventionally, a permanent magnet that has been already magnetized alone is used to assemble a rotor. In such a method, however, since the permanent magnet attracts iron powder and dust during the assembling, the assembling of an electric rotating machine becomes difficult, and the reliability of the completed electric rotating machine decreases.